1. Field of the Invention
The present invention relates to a direct type fuel cell in which fuel and air are made to react chemically with each other to produce electrical energy.
2. Description of the Related Art
It is well known that there are provided fuel cells in which fuel and air are made to react chemically with each other to produce electrical energy. Of those fuel cells, a direct type fuel cell can be made smaller, and is expected to serve as an energy source for a portable electronic device such as a cellular phone or a notebook computer.
FIG. 13 is a vertical sectional view of a conventional fuel cell B.
As shown in FIG. 13, the conventional fuel cell B comprises an electromotive layer 100, a fuel supplying layer 200 and an air supplying layer 300. The electromotive layer 100 is provided to produce electrical energy. The fuel supplying layer 200 is provided on a surface of the electromotive layer 100 to supply fuel to the electromotive layer 100. The air supplying layer 300 is provided on another surface of the electromotive layer 100, which is located opposite to the above surface, to supply air (oxygen) to the electromotive layer 100.
The electromotive layer 100 comprises an electrolyte membrane 100a, a plurality of fuel poles 110b, and a plurality of oxidizer poles 100c. In the electrolyte membrane 100a, fuel and air are made to react chemically with each other. The fuel poles 100b are arranged in a matrix on a surface of the electrolyte membrane 100a, which is close to the fuel supplying layer 200, and supply fuel from the fuel supplying layer 200 to the electrolyte membrane 100a. The oxidizer poles 100c are arranged in a matrix on another surface of the electrolyte membrane 100a, which is located opposite to the above surface and close to the air supplying layer 300 of the electrolyte membrane 100a, and supply air from the air supplying layer 300 to the electrolyte membrane 100a. 
The fuel poles 100b and the oxidizer poles 100c are located opposite to each other, with the electrolyte membrane 100a interposed between them. The fuel poles 100b, the oxidizer poles 100c and the electrolyte membrane 100a form respective electromotive portions 110 for generating voltages. That is, the electromotive layer 100 comprises the electromotive portions 110 the number of which is equal to each of that of the fuel poles 110b and that of the oxidizer poles 100c. The electromotive portions 110 are all connected in series to each other. The sum of the voltages produced by the electromotive portions 110 is the final output voltage of the fuel cell B.
The fuel supplying layer 200 comprises a felted liquid holding sheet 200a for supplying fuel to the fuel poles 100b, a replenishing portion 200b connected to the liquid holding sheet 200a, for replenishing the liquid holding sheet 200a with fuel, and a fuel diffusion film 200d which permits the fuel in the liquid holding sheet 200a to pass through the fuel diffusion film 200d, and which is provided on a surface of the liquid holding sheet 200a, which is close to the electromotive portions 110.
The air supplying layer 300 comprises a felted moisture-retentive sheet 310 which takes therein air from the vicinity of the fuel cell B, and then supplies it to the oxidizer poles 100c, and which prevents releasing moisture which is produced in the electrolyte membrane 100a due to chemical reaction of fuel and air, i.e., prevents drying of the electrolyte membrane 100a. 
In the fuel cell B having the above structure, fuel with which the liquid holding sheet 200a is replenished from the replenishing portion 200b is advanced within the liquid holding sheet 200a toward the fuel diffusion layer 200d, while diffusing in a direction away from the replenishing portion 200b. It then reaches the fuel poles 100b on the fuel diffusion film 200d. 
On the other hand, air taken in the moisture-retentive sheet 310 from the vicinity of the fuel cell B passes through the moisture-retentive sheet 310, and reaches the oxidizer poles 100c on the moisture-retentive sheet 310.
Then, the fuel reaching the fuel poles 100b and the air reaching the oxidizer poles 100c react chemically with each other at the electrolyte membrane 100a, causing voltages to be produced between the fuel poles 100b and the oxidizer poles 100c. The sum of voltages produced at the electromotive portions 110 is the final output voltage of the fuel cell B.
Furthermore, in a direct type fuel cell disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2000-106201, a plurality of electromotive layers are stacked together, and liquid fuel is supplied to the electromotive layers by capillary action.
In most conventional fuel cells, the replenishing portion is provided at an end of the liquid holding sheet since the replenishing portion is restricted by the structure of the cell. In such a structure, there is a case where the concentration of the fuel in the liquid holding sheet is not uniform, i.e., the concentration varies from one part of the liquid holding sheet to another. In this case, the concentrations of fuel reaching the electromotive portions are also different, thus worsening the efficiency of production of electrical energy by the electromotive portions.
For example, if a large amount of fuel is supplied to the electromotive portions, surplus fuel which has not reacted in the electrolyte membrane reaches the oxidizer poles. Thus, it is wasteful, and in addition the voltage loss may increase due to reduction of the surface areas of catalysts included in the oxidizer poles. On the other hand, if an excessively small amount of fuel is supplied to the electromotive portions, the reaction energy becomes too large, as a result of which the voltage loss may also increase.
Thus, in order to improve the electrical characteristics of the fuel cell, it is important to make the concentration of fuel in the entire liquid holding sheet uniform, and supply a proper amount of fuel to the electromotive portions. However, in a portable electronic device required to operate for a long time, a fuel cell including a large liquid holding sheet is provided, and the concentration of fuel therein greatly varies from one part to another. Therefore, as is often the case, the amount of fuel supplied to each electromotive portion is too large or small, thus remarkably worsening the electrical characteristics.